Blender (Cycles) Manual DOF
One way to do this:


Set the depth of field to your liking and keep the focus on the object you want sharp, even if adjacent objects on the same focal plane look sharp.
Separate the object in focus in one layer and the rest a different layer.
Assign the layers to different render layers.
On the compositor choose the render layer with the objects you want to defocus, and add a blur node. 
Mix the two layers back together.


If you want a bit finer control, use a defocus node instead of blur and control the blur using the Z information.

EDIT

Here's another solution to make "impossible" Depth of Field. Using a color ramp to modify the Z pass on a defocus node.This way you have absolute control on what goes in focus and how blury it gets!




The blend file is here: